big ships need two of them, what about small ships? do they remove one token at a time and so the effects last two moves or are both removed after first move? Is there a limit of Ion tokens a ship can 'hold'?

TIA

All Ion tokens are removed after the white one straight move. All of them.

The number of ion tokens on a ship is not limited.

Every time a small ship is hit by an ion you suffer 1 damage and place an ion token next to it. During your next movement phase you "drift/float" forward using the 1 straight (considered a white manuever). After that clear all ion tokens - you are no longer affected.

With a large ship you still suffer 1 damage for each ion hit like with small ships. The big difference is that you do not suffer the "drift/float" effect until you are assigned a second ion token. A single ion token stays with you until you are assigned a second. Once the second one has been assigned perform a 1 straight white during your next movement phase and then clear all ion tokens (even if you've actually taken 3).

Pretty sure I've got that all correct.